1. Field of the Invention
The invention relates to a method of and arrangement for transmitting messages between a fixed radio station and a plurality of mobile radio stations in a digital radio transmission system, and particularly to such a system wherein transmission by the base station is by code division multiplex modulation and transmission by the mobile stations is by frequency division multiplex modulation.
2. Description of the Related Art
For the transmission of messages via a transmission means (for example lines, radio paths) used in common by a plurality of subscribers, three basic methods are known, namely the code-division multiplex method, the frequency-division multiplex method and the time-division multiplex method.
In the code-division multiplex method the different messages to be conveyed over a common transmission channel are, for example, modulated on a carrier by means of basic modulation and the resultant signal, which has a narrow band compared with the general bandwidth, is spectrally distributed over the channel bandwidth by multiplex modulation with the aid of a code word identifying the receiver. The code multiplex channel (message transmission channel) thus obtained is not limited in time or bandwidth, but is limited as regards its power density. Recognition of the signal is not effected by time selection or on the basis of frequency, but on the basis of the spectral code. The superposed plurality of such spectrally encoded messages in the code multiplex channel are selected in the receiver on the basis of the code word assigned to it. For the two-stage modulation (basic and multiplex modulation), phase shift keying (PSK) or frequency shift keying (FSK) are frequently used in radio transmission systems.
The digitised speech signal at the transmitter end is, for example, applied (after A/D conversion) to the first stage which includes, for example, a multiplicative mixer. In the multiplicative mixer the applied digitised speech signal is combined with a code word assigned to this transmitter, which results in a spectral distribution. In the second modulation stage of the transmitter the wide-band signal (modulated, binary character sequences) are coverted to a frequency position appropriate for the transmission.
Recovering the message at the receiver end is effected in the above-described code-division multiplex method by the sequence of base-demodulation and multiplex-demodulation. In the base-demodulation stage the conversion to a frequency position (for example base-band position) appropriate for the multiplex-demodulation is effected by multiplying the signal by the reference carrier. With the aid of a code word generator arranged in the receiver and also a code-synchronising circuit, the spectral distribution is cancelled, after the code word generator has been synchronised in the correct phase with the reception code word. As a result thereof the signal energy which was previously spectrally distributed over the overall transmission band, is compressed back into the original narrow frequency band, whilst the adjacent signals reaching the receiver by means of a different multiplex modulation remain spectrally distributed and can be suppressed using a bandpass filter having a bandwidth which corresponds to the bandwidth of the non-distributed signals.
The system-determined residual noise still remaining in the multiplex demodulation and caused by the other signals is smaller according as the values of the cross-correlation functions between the code words used are lower and the distribution factor is greater. A value different from zero of the cross-correlation function reduces the signal-to-noise ratio. The signal-to-noise ratio and the synchronising period are determined by the cross-correlation and autocorrelation functions.
In the frequency-division multiplex method the total bandwidth available for the transmission of messages is divided into narrow frequency bands which each correspond to a message transmission channel. For the duration of the radio transmission the subscriber has such a narrow frequency band at its disposal.
In the time-division multiplex method each subscriber has the disposal of the total bandwidth of an individual radio channel, which the subscriber may use only for short time intervals. The characters or character sequences of different subscribers are interleaved and are transmitted at a correspondingly higher bit rate in the individual radio channel, the time channel assigned to each subscriber being repeated periodically at the frame period intervals.
West German Patent DE-OS 25 37 683 discloses a radio transmission system with stationary radio stations and mobile radio stations, in which different channel-accessing methods with asynchronous time multiplex, code multiplex and frequency multiplex are used.
In a radio transmission system connections are effected via repeatedly reflected propagation paths containing obstacles. The low effective aerial height and the necessary circular radiation characteristic of the aerials of mobile radio stations effect that the receiving field strength is exclusively composed of the interference from sub-field strengths, which are produced by multipath propagations as a result of reflections from the earth, mountains, buildings, etc. The receiving voltage pulled in by the aerial is additionally amplitude and phase-modulated by superpositioning of the sub-field strength. The reflected signal components reach the aerial with different time delays via different paths. The influence of the multi-path propagation is particularly noticeable when the delay time difference between the direct and the reflected signals is more than half the duration of a code word character in the signal. The signal mix produced by this multi-path propagation results in location-dependent amplitude distributions. In the range of small receiving field strengths the minima of this amplitude distribution result in position-dependent values falling short of the limit sensitivity, and consequently result in disturbed connections. For stationary vehicles the fluctuation in the received voltage is comparatively low. If the vehicle moves, the noise spectrum is significantly more pronounced. The disturbances in the connection, whose duration and rate of occurrence depend on the speed and correspond to a Rayleigh distribution, are based on a field strength distribution which depends on the route taken by the vehicle and depends on the reflection coefficients of the environment. In the case of disadvantageous propagation circumstances or high vehicle speeds, error rates of much more than 1% can be measured, which for a brief period of time reach approximately 50%.
It has been found that because of the system-determined noise the code-division multiplex method has a smaller signal-to-noise ratio than the time-division multiplexer method or the frequency-division multiplex method. However, because of the spectral compression of the message energy the signal-to-noise ratio in the receiver improves in accordance with the distribution factor (number of code word characters per message character). During the transmission of the message from the mobile radio station to the stationary radio station the receiving energy in the stationary radio station is lower according as the distance to the mobile radio station is longer. Differences in distance by a factor of 10 result, on an average, in energy differences of 40 dB, so that each mobile radio station requires a transmission power control for the necessary dynamic compensation. In addition to the disadvantage that a larger bandwidth is required, the code-division multiplex method has, compared with the time-division multiplex or frequency-division multiplex method, the further disadvantage that the code synchronisation is significantly more expensive. On the other hand, no adjacent channel problems occur in the code-division multiplex method, as may be the case in the frequency-division multiplex method because of overlapping frequency bands, or in the time-division multiplex method because of overlapping time slots. Because of the spectral "distribution" of the transmission energy, wide-band noise sources and also other noise sources result in code words superposed on the sum signal, so that a code-division multiplex system is inferior to other systems as regards frequency.
The disadvantage of requiring a larger bandwidth is compensated for in a radio transmission system by the advantage that the same frequency band can be utilized in spatially adjacent radio cells, so that the overall bandwidth-assuming that a rather large number of molbile radio stations are permitted in the radio transmission system--is of the same order of magnitude as the overall bandwidth in the frequency-division multiplex method. Because of the same frequency band it is easy to implement a radio cell exchange by the mobile radio station.
The code-division multiplex method has, compared with the frequency-division multiplex method, the advantage that, using the same code word, signalling information components can be radiated simultaneously to all the mobile radio stations. However, the code-division multiplex method, because of the significantly fluctuating differences in the distances between mobile and stationary radio stations determined by the large attenuation differences during the wave propagation, has the disadvantage that it requires a power control in each mobile radio station. The time-division multiplex method has the disadvantage that--in order to avoid to a significant extent the cancellations of modulation in binary encoding--only a maximum bit rate of approximately 50 kbit/s can be accomplished. This maximum bit rate is however not sufficient to provide a large time-interleaing of the signals from a plurality of subscribers, as this quantity of information corresponds already to the quantity of information from only a few subscribers (2-3 subscribers).
Combinations of the above-described methods and their use in a digital radio transmission system are also known. For example "Nachrichtentechnik, Elektronik+Telematic 38 (1984), Vol. 7, pages 264 to 268" discloses a digital radio transmission system in which the time-division multiplex method is used in combination with the code-division multiplex method. In the time channels for transmitting speech and/or data (communication channel TCH) there are transmitted, one after the other, a bit sequence for determining the bit clock rate (synchronous), a frame-synchronising word (leader) and the bit sequence of the message itself. The time channels for the transmission of messages (3.times.20 TCH) are combined with control channels (3 CCH) to form a time-division multiplex frame having a duration of 31.5 msec. If the speech signal is to be transmitted as a message, then the adaptive delta modulation method can be used for analog/digital conversion. The message characters (bits) then produced are superposed by a code in a transmitter. It was found to be advantageous to combine individual message characters in blocks of four bits each and to distribute the blocks thus obtained with an orthogonal alphabet. The distribution factor then used is a compromise, to make it possible to combine the advantages of the band distribution with the requirements as regards the economic use of frequency.
All the radio transmission systems known so far have in common that the message transmission channels between stationary and mobile radio stations are of a symmetrical structure, that is to say the same modulation method is used for the forward and return directions. So far no investigations have been made to investigate whether, when each time a different modulation method is used for the forward and return directions of the message transmission channels the above-described disadvantages can be avoided.